From: University of Massachusetts
Posted: Tuesday, April 24, 2007
AMHERST, Mass. -- A fender-bender between two celestial bodies that left a 200 mile-wide metallic chunk to cool in space was the likely source of a group of meteorites known as the IVA iron meteorites, suggests new research by University of Massachusetts Amherst scientists. Their findings, published in the April 19 issue of the journal Nature, help explain conflicting meteorite data that has long puzzled scientists, and sheds new light on how and when asteroids form.
Jijin Yang and Joseph Goldstein of the UMass Amherst department of mechanical and industrial engineering, and Edward Scott of the Hawaii Institute of Geophysics and Planetology at the University of Hawaii at Manoa collaborated on the research.
The standard model of asteroid formation says asteroidal bodies are just leftover debris from the collisions and subsequent melting that happens when planets form. Scientists find that these leftover chunks typically have a dense iron core containing nickel, surrounded by an insulating layer of silicate. Evidence has suggested that the iron-nickel core cools relatively evenly, thanks to the insulating silicate mantle.
But when researchers have calculated cooling rates for the 60-odd meteorites that are known as the IVA iron meteorites (believed to have come from a single parent asteroid), they've gotten wildly different numbers, says Goldstein.
"We find that these cooling rates of the IVA irons vary by a factor of more than 50 and directly with the nickel content of the iron meteorite," he says. "This means there's something goofy happening."
Given the insulating silicate mantle, the cooling rates of the IVA irons ought to have been very similar, he says. So Goldstein and his colleagues re-calculated cooling rates for 10 IVA irons and combined the data with computer model simulations. They also examined the microstructure of several of the irons using a transmission electron microscope.
The IVA meteorites must have cooled as one, big chunk, roughly 200 miles-wide and without an insulating mantle, the scientists conclude, not in the form of a smaller insulated body as had previously been thought. If correct, the parent asteroid would have been comparable in size to the largest M class asteroid, 6 Psyche, says the research team.
"You can see the same phenomenon occurring when cooling steel," explains Goldstein. "If you take a new piece of steel out of a huge blast furnace and set it down, we know that the outside cools a lot faster than the inside because there's no insulation. The same would be true of the IVA irons in a metallic asteroid."
Roughly 60 meteorites retrieved from around the world have the chemical makeup of the IVA irons, suggesting that they were all part of one metallic asteroid that broke up about 450 million years ago and then fell to earth in pieces.
Scientists have proposed several theories over the decades to rationalize the varying cooling rates seen in IVA meteorites. One is that either the data or the computer simulations are faulty. There's the "Rubble-Pile" model, which posits that various pieces of the asteroid broke off at some point and then were thrown back together by influences such as gravity and centrifugal force. Another model, the so-called "Raisin Bread" effect, explains the various cooling rates by picturing various metal chunks spread throughout the silicate mantel of the asteroid. None however, could explain why the cooling rates vary directly with the nickel content of the meteorites.
Now the researchers think the IVA irons' parent asteroid must have formed after two protoplanets sideswiped each other, thus breaking off many different pieces with varying amounts of silicate. The authors believe that the metal containing the IVA irons was one of these pieces that contained little or no silicate insulation.
"Our study was created to understand how the asteroid was formed almost one million years after the formation of the solar system," says Yang. "Our theory explains the different cooling rates as part of a comprehensive description of the formation of the asteroid containing the IVA irons."
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